Semiconductor technology has made such a remarkable progress up to now that miniaturized electronic control circuits operable with an improved functional characteristic and having increased integration density can be mass-produced at low cost, and such electronic control circuits have come to be widely used in both domestic electrical appliances and industrial applications.
In various heating apparatus including electric ovens, microwave ovens, gas ovens and hybrids of these ovens, there has been rapid progress in the development of intelligent electronic control circuits. An especially marked tendency in heating apparatus of the kind above described has been the use of various sensors for sensing the condition of an object being heated thereby automatically controlling the process of heating, and such automatic heating apparatus have very quickly penetrated the market.
Such automatic heating apparatus has gained popularity because the control part responsing to the output of the sensor acts to automatically end the heating sequence in contrast to earlier types in which the user had to manually set the factors including the duration of heating, heating output and heating temperature. Therefore, in a heating apparatus such as a microwave oven in which the factors including the quantity of an object to be heated and the initial temperature must be taken into consideration for cooking, it has become possible to very conveniently handle the oven and to attain desired heating with the least possibility of failure.
An example of such a prior art apparatus is disclosed in Japanese Patent Lay-Open publication No. 51-134951 (1976). In the automatic heating apparatus disclosed in the cited patent application, a so-called humidity sensor senses continuously variations of the relative humidity in the heating cavity resulting from progressive emission of water vapor from an object being heated, until finally a vapor sensing point is reached at which the relative humidity attains a predetermined setting. According to the disclosure, the heating period of time T.sub.1 elapsed until the vapor sensing point is reached is added to the product kT.sub.1 obtained by multiplying T.sub.1 by a separately determined coefficient k peculiar to the object to be heated. These values are used to calculate the sum (T.sub.1 +kT.sub.1) which is determined to be the total duration of heating required for satisfactorily cooking the object.
Although the above description refers merely to the control of automatic heating by the use of the so-called humidity sensor, this control method is also very effectively applicable to the control of automatic heating by the use of a so-called gas sensor which reacts with water vapor, alcohol and CO.sub.2 gas. However, the disclosed control method has been disadvantageous in that the process of heating is ended before the temperature of an object to be heated has increased sufficiently. That is, the so-called "premature ending of heating" tends to occur, unless the object is made gastight by covering it with a sheet such as a plastic sheet or enclosing it in a lidded container.
FIG. 3 is a graphic representation of such a situation. More precisely, FIG. 3 shows variations, relative to time, of the relative humidity in the heating cavity. It will be seen in FIG. 3 that the relative humidity in the heating cavity decreases gradually immediately after starting of the process of heating due to a gradual rise of the internal temperature of the heating cavity, and, then, when water vapor starts to emit from an object being heated, the relative humidity in the heating cavity shows a sharp increase. In the example, shown in FIG. 3, the object to be heated is water, and the source of heating energy is a magnetron. The solid curve H.sub.1 in FIG. 3 represents the case in which a container filled with water is covered with a plastic sheet, and the dotted curve H.sub.2 represents the case in which the container is not covered with such a sheet. The temperature of water at the end of the process of heating is shown at the right-hand shoulder portion of each of the curves H.sub.1 and H.sub.2. The initial temperature of the water was 20.degree. C. in each of these cases. Comparison between the curves H.sub.1 and H.sub.2 makes it clear that the temperature of the water at the end of the process of heating is lower in the case of the curve H.sub.2 than in the case of the curve H.sub.1.
It will be seen in FIG. 3 that, in the case of the curve H.sub.2 which represents the relative humidity when the object is heated without the cover, the value sensed by the sensor attains a predetermined setting at a point P.sub.2 at which partial vaporization starts, resulting in the "premature ending of heating". In contrast, in the case of the curve H.sub.1 which represents the relative humidity when the object is heated in the covered state, water vapor and gas are not emitted into the heating cavity from the object until the vapor pressure in the covered container builds up to a certain level. Consequently, the emission of water vapor and gas from the object is sensed at a point P.sub.1 which is much later in time than the point P.sub.2 and the object can be heated up to a sufficiently high temperature.
It has thus been difficult to effect failure-free heating unless the presence or absence of a cover is specified. By the way, in the case of, for example, reheating of a cooked foodstuff, there is a strong user demand for reheating the cooked foodstuff either in a covered condition or in a non-covered condition depending on the kind of cooked foodstuff to be reheated. In the case of the reheating above described, a better result can be expected when a cooked foodstuff such as fried chicken or rice is reheated without the use of a cover or a lidded container than when it is reheated in a covered or lidded condition. This is because a crisp finish is desired for such a cooked foodstuff. When, on the other hand, a cooked foodstuff such as a boiled or steamed foodstuff is reheated without the use of a cover or a lidded container, it will be excessively dried, resulting in failure of satisfactory reheating.
The same applies also to the cooking of a raw foodstuff. Generally describing, it is important to cook it without a cover when a crisp finish is desired and to cook it with a cover when a wet finish is desired.
The above problem can naturally be solved by arranging more keys on the keyboard of the automatic heating apparatus. However, the user will feel that the selection of a desired key is troublesome when many keys including such additional keys are arranged on the keyboard. That is, the user must select either "REHEATING (WITH COVER)" or "REHEATING (WITHOUT COVER)". The number of required keys is two times as many as that required hitherto, and an input circuit of complex structure is naturally required resulting in an increase in the cost.
Keys specifying the presence and absence of a cover may be provided and manipulated to select a required heating sequence after selection of a menu. However, the number of times such keys have to be manipulated will increase, and the possibility of manipulation will inevitably become high. Anyway, the method of changing over the heating sequences by manipulation of such keys cannot remedy the case in which an object to be heated is loosely covered, giving rise to "premature ending of heating" or the case in which, in spite of the use of a lid covering a container, the result of cooking tends to differ depending on the size of the lidded container.